Bulky unnatural amino acids, such as diarylalanines, are of interest since these compounds may serve as surrogates for their natural counterparts. Since the aromatic rings of phenylalanine and tyrosine amino acid residues often play important roles in peptide-receptor interactions, replacement of one of these residues with a bulky diarylalanine residue, e.g., diphenylalanine, has the potential to dramatically enhance the therapeutic activity of peptide analogs. Incorporation of a diarylalanine into a bioactive peptide may also impart biostability by inhibiting degradation by peptidases and/or provide conformational restriction. Either effect can serve to increase the pharmacological activity of a polypeptide and increase its potential as a therapeutic agent. For example, a peptidyl antagonist which incorporates an optically active form of a diarylalanine, the D-enantiomer of diphenylalanine (D-DIP), is known to be a potent antagonist of the endothelin ET.sub.A and ET.sub.B receptors. A number of luteinizing hormone-releasing hormone analogs containing various hydrophobic unnatural amino acid substitutions have been reported to have potent biological activity. Hydrophobic peptides which include a 3,3-diphenylalanine residue have been reported to have antihypertensive activity.
A series of angiotensin II analogs in which the phenylalanine at the 8-position was replaced with various unnatural amino acids including 3,3-diphenylalanine have also been described. The authors of this study indicated that they were unable to resolve the 3,3-diphenylalanine enzymatically using either carboxypeptidase or hog kidney acylase. In order to obtain the desired angiotensin II analog it was necessary to separate via countercurrent distribution the diastereomeric peptides which had been prepared from racemic 3,3-diphenylalanine. While this permitted the desired analog to be obtained, the approach was extremely inefficient since half of the product produced was the undesired diastereomer.
In order for unnatural amino acids such as diphenylalanine to become effective building blocks for the design of peptide analogs, methods which permit the unnatural amino acids to be readily prepared in high yield and in optically active form must be available.
A number of preparations of diphenylalanine (DIP) in racemic form have been reported. The unnatural amino acid has been prepared through alkylation of an acetamidomalonate ester or a hindered imine of a substituted glycinamide. A derivative of DIP has also been produced through an azide addition to a 3,3-diphenylpropionamide. Other routes, such as the alkylation and subsequent reduction of a nitroacetic acid ester are also known. These routes have not provided access to the optically active forms of DIP, nor have they facilitated the preparation of a wide variety of analogs with differing aryl groups.
Several unsuccessful attempts to resolve diphenylalanine by enzymatic resolution of a racemic derivative have been reported. These include unsuccessful attempts to selectively hydrolyse N-BOC diphenylalanine using papain or .alpha.-chymotrypsin. An effort to selectively hydrolyse N-acetyl DIP using hog kidney acylase or carboxypeptidase was also reported to be unsuccessful.
Both the D- and L- isomers of DIP have been obtained from the racemate by conventional resolution using cinchona alkaloids. This technique however, which requires the repeated recrystallization of an alkaloid salt, is not especially attractive for the preparation of large quantities of the unnatural amino acid.
More recently, there have been several reports of the preparation of DIP via asymmetric synthesis. These reports include asymmetric alkylation of a sultam-derived glycine imine, stereoselective alkylation of a hindered glycine imine in the presence of a cinchona-based phase transfer catalyst, and asymmetric azide addition to a 3,3-diphenylpropionamide using chiral auxiliary methodology. The chiral auxiliary based methods require extra steps for the introduction and cleavage of the chiral auxiliary, which is typically incorporated into a precursor as part of an amide derivative. As with the chiral phase transfer method, the chiral auxiliary methods include a purification step to remove the agent which confers chirality.
All of the methods described above suffer from one or more of a number of disadvantages--low yields, general difficulty in scaling up the procedure, use of costly reagents, inclusion of extra reaction or purification steps to introduce and/or remove a chiral agent, and the inability to alter the preparation to provide a variety of related derivatives. Accordingly, in view of the potential utility of diarylalanines in the preparation of peptide analogs and other compounds of pharmaceutical interest, there is a continuing need for methods which would permit the efficient, large scale preparation of a variety of diarylalanines and if desired, in optically active form.